1. Technical Field
The present disclosure relates to an abnormality detection device for a continuously variable transmission and a method of detecting abnormality of the continuously variable transmission.
2. Related Art
These days, continuously variable transmissions (CVT) such as a chain-type CVT or a belt-type CVT, which allow a gear ratio to be continuously changed, are widely in practical use (see, for example, Japanese Unexamined Patent Application Publication (JP-A) No. 2001-225672). In the continuously variable transmission described in JP-A No. 2001-225672, in order to suppress the degradation of power characteristics and a sense of discomfort due to a difference in responsiveness between the engine and the continuously variable transmission, transmission control of the continuously variable transmission is performed according to a target output which is calculated based on an accelerator opening and a vehicle speed. On the other hand, smoothing processing is performed on the target engine torque which is calculated from the target output, and the output of the engine is controlled based on the target engine torque which has undergone the smoothing processing.
While, as described above, the engine output torque is limited in order to absorb the difference in responsiveness between the engine and the continuously variable transmission, the engine output torque is also limited, for example, in order to protect the continuously variable transmission (for example, to prevent the chain from sliding, to prevent the clutch from being damaged, and/or to prevent input of torque greater than or equal to an allowable torque).
In general, the continuously variable transmission is controlled by an electrical control device using a microcomputer, for example. Although it is extremely rare for such an electrical control device to have a failure, a fault might occur in the microcomputer, for example, a fault in a memory such as a RAM and a register (for example, written data and read data do not match), a fault in a logical operation circuit, and variation in clock signal might occur. In addition, an error in setting or writing ROM data (such as control data) might occur.
If such a hardware related fault and/or an error in setting ROM data occurs, the above-described upper limit (limiting value) of torque output from the continuously variable transmission may have an abnormal value, the upper limit limiting the engine output torque. For example, in the case where the upper limit of torque has an abnormal value (for example, substantially zero) while the vehicle having such abnormality is being accelerated after an accelerator pedal is depressed, the vehicle is decelerated against the intention of the driver, that is, despite of the intention of accelerating the vehicle.
Thus, an occurrence of a failure as described above, a workaround for the failure is necessary such that control over the continuously variable transmission and the engine is not adversely affected. However, in the continuously variable transmission described in JP-A No. 2001-225672, detection of abnormality (determination of reasonableness) of the upper limit (limiting value) of torque, which limits the engine output torque, is not taken into consideration, the upper limit of torque being requested for the engine by the continuously variable transmission. Even when the upper limit of torque is same, the vehicle may be decelerated or may not be decelerated depending on, for example, a gear ratio, and thus it is not possible to determine whether or not the upper limit of torque is abnormal only by the value of the upper limit.